Liquid crystal display devices have become widely used in various fields in televisions, monitors, mobile phones, and the like. These devices offer characteristics such as energy saving, thinness, and being light-weight.
These types of liquid crystal display devices are categorized as transmissive, reflective, or transflective, depending on the light source used for display.
Transmissive liquid crystal display devices perform display by a liquid crystal display panel provided in a liquid crystal display device being illuminated with light from a backlight, which is provided separately. This allows for a bright, high-contrast display, but has high energy consumption.
On the other hand, reflective liquid crystal display devices do not use light from a backlight to perform display, but rather reflect ambient light through a reflective electrode disposed in the liquid crystal display panel. This does not require a backlight, and as such can suppress power consumption, but the contrast will drop depending on the ambient brightness where the reflective liquid crystal display device is used.
To improve on these issues with transmissive and reflective liquid crystal display devices, a transflective liquid crystal display device has been developed that has, in a single pixel of the liquid crystal display panel, a transmissive region for performing display with light from the backlight, and a reflective region that performs display with ambient light reflected by the reflective electrode.
In this type of transflective liquid crystal display device, the transmissive region performs display with light from the backlight when the ambient environment is dark, and thus is capable of maintaining a certain level of high contrast without relying on the surrounding brightness.
Furthermore, in this transflective liquid crystal display device, the reflective region performs display with the ambient light reflected by the reflective electrode, and the reflective region does not use light from the backlight, which makes it possible to achieve a reduction in power consumption.
A transflective liquid crystal display device having these characteristics is used indoors or outdoors and is actively adopted into mobile devices such as smartphones and mobile phones that have a limited power supply.
Transflective liquid crystal display devices, however, have problems as described below.
FIG. 18 is a schematic configuration of a conventional transflective liquid crystal display panel.
As shown in FIG. 18(a), a transflective liquid crystal display panel 100a includes an active matrix substrate, color filter substrate, and a liquid crystal layer 107 constituted of liquid crystal molecules 106 sandwiched between these two substrates.
The active matrix substrate has an insulating substrate 101 that allows visible light to pass therethrough, TFT devices (not shown) formed on the insulating substrate 101, an insulating layer (not shown), a reflective electrode 102 and a transparent electrode 103 as a pixel electrode connected to a drain electrode of the respective TFT devices, and an alignment film (not shown).
The color filter substrate has an insulating substrate 104 that allows visible light to pass therethrough, a transparent electrode 105 as a common electrode, and an alignment film (not shown).
As shown in FIG. 18(a), the reflective electrode 102, which is a portion of the pixel electrode, is disposed in the reflective region of the transflective liquid crystal display panel 100a, and the transparent electrode 103, which is another portion of the pixel electrode, is disposed in the transflective region.
Merely providing the reflective electrode 102 in the reflective region of the transflective liquid crystal display panel 100a and the transparent electrode 103 in the transparent region in this manner will cause the light in the reflective region and the transmissive region to be in different phases.
This phase difference is a value determined by the birefringence Δn of the liquid crystal layer and the panel gap (thickness of liquid crystal layer) d.
The reason that a difference in the phases of light in the reflective region and transmissive region would occur is that, in general, the distance that light travels through the liquid crystal layer 107 in the reflective region is two times that of the transmissive region in the transflective liquid crystal display panel 100a having both the transmissive region and the transflective region.
In other words, the optical path is one-way in the transmissive region, but two-way in the reflective region.
Accordingly, in the transflective liquid crystal display panel, phase of light in the reflective region and the phase of light in the transmissive region need to be made the same in order to approximately equalize the optical characteristics of the transmissive region and the reflective region.
Liquid crystal display panels 100b and 100c that have had the optical characteristics of the transmissive region and the reflective region thereof approximately equalized in this manner are shown in FIGS. 18(b) and 18(c).
In the liquid crystal display panel 100b shown in FIG. 18(b), a multi-gap configuration is adopted in which a panel gap adjusting structure 108 is provided in the reflective region in order to make the phase of light in the reflective region the same as the phase of light in the transmissive region and to reduce the panel gap (thickness of the liquid crystal layer) d of the reflective region to one half of the panel gap (thickness of the liquid crystal layer) d in the transmissive region.
In this configuration, however, it is necessary to provide a structure with recesses and protrusions on the substrate, which can make the structure become complex and require more precision in the manufacturing process.
Meanwhile, the liquid crystal display panel 100c shown in FIG. 18(c) has an electrode structure in which different electric fields A and B can be respectively applied to the reflective region and the transmissive region in order to make the orientation state of the liquid crystal molecules 106 in the reflective region and transmissive region different from each other. The respective applying of the different electric fields A and B to the reflective region and the transmissive region is driven by a driving method.
In this structure, however, it is necessary to have this complicated driving method and electrode structure, which poses problems.
In the commonly used modes for liquid crystal display devices, such as VA mode, IPS mode, and FFS mode, due to the reasons described below it is necessary to use the panel gap adjusting structure 108 and the driving method to respectively apply the different electric fields to the reflective region and transmissive region described above in order to make the phase of light in the reflective region and the phase of light in the transmissive region the same, thereby approximately equalizing the optical characteristics of the transmissive region and the reflective region.
This is because in most VA modes, where the liquid crystal molecules are oriented perpendicular to the substrate surface when there is no applied voltage and then twisted when there is applied voltage in order to perform display, the orientation of the liquid crystal molecules when there is applied voltage is substantially uniform along the horizontal direction of the substrate due to a change in the orientation direction caused by a uniform electric field generated between the two substrates (horizontal flat plates). Therefore, there is not much change in the birefringence Δn of the liquid crystal layer along the horizontal direction of the substrate, and thus, in order to make the phase of light in the reflective region the same as the phase of light in the transmissive region, it is necessary to make the birefringence Δn of the liquid crystal layer in the reflective region and transmissive region different from each other by either adjusting the panel gap (thickness of the liquid crystal layer) d using the panel gap adjusting structure 108 or by respectively applying different electric fields to the reflective region and transmission region.
In most IPS mode or FFS mode liquid crystal display devices, which perform display by twisting the liquid crystal molecules on the substrate surface, the liquid crystal molecules are oriented horizontally to the substrate surface when there is no applied voltage; therefore, it is difficult to achieve a normally black mode liquid crystal display device even if the same circularly polarized light in the above mode is used for VA mode transflective liquid crystal display devices.
Accordingly, in IPS mode or FFS mode liquid crystal display devices, to obtain a normally black mode liquid crystal display device using circularly polarized light, it is necessary to use the panel gap adjusting structure 108 or the driving method that respectively applies different electric fields to the reflective region and the transmission region, as described above.
Therefore, in a VA mode, IPS mode, or FFS mode liquid crystal display device, the structure will become complicated if circularly polarized light is used to obtain the normally black mode liquid crystal display device.
There is also a proposal for a normally black mode transflective liquid crystal display device that uses a mode other than the VA mode, IPS mode, or FFS mode, and that makes the optical characteristics of the transmissive region and the reflective region approximately equal by making the phase of the light in the reflective region the same as the phase of the light in the transmissive region without using the panel gap adjusting structure 108 or the driving method that respectively applies different electric fields to the reflective region and the transmissive region.
Patent Document 1 discloses a configuration in which the phase of light passing through the reflective region and the phase of light passing through the transmissive region are made uniform by using a comb-shaped electrode and a TBA mode (transverse bend alignment mode) that performs display by liquid crystal molecules that are perpendicular in an OFF state when there is no applied voltage being oriented to the horizontal direction in a vent shape by using a horizontal electric field in an ON state when there is applied voltage.